Molecular Monodisperse polymer

Polymerization ofiVIasked Disilenes. A novel approach, namely, the anionic polymerization of masked disilenes, has been used to synthesize a number of poly(dialkylsilanes) as well as the first dialkylamino substituted polysilanes (eq. 13) (111,112). The route is capable of providing monodisperse polymers with relatively high molecular weight M = lO" — 10 ), and holds promise of being a good method for the synthesis of alternating and block copolymers. 

A further feature of anionic polymerisation is that, under very carefully controlled eonditions, it may be possible to produee a polymer sample which is virtually monodisperse, i.e. the molecules are all of the same size. This is in contrast to free-radical polymerisations which, because of the randomness of both chain initiation and termination, yield polymers with a wide molecular size distribution, i.e. they are said to be polydisperse. In order to produce monodisperse polymers it is necessary that the following requirements be met ... 

Plate count should always be tested with a monodisperse sample of low molecular weight. Polymers can also be used, but they show much lower plate counts because their diffusion coefficients are much smaller than those of low molecular weight compounds. 

According to the concepts, given in the paper [7], a significant difference between the values of yield stress of equiconcentrated dispersions of mono- and polydisperse polymers and the effect of molecular weight of monodisperse polymers on the value of yield stress is connected with the specific adsorption on the surface of filler particles of shorter molecules, so that for polydisperse polymers (irrespective of their average molecular weight) this is the layer of the same molecules. At the same time, upon a transition to a number of monodisperse polymers, properties of the adsorption layer become different. 

Fig. 46. Dependence of the degradation yield on strain rate e(0), and on ratio of weight- to number-average molecular weight (Mw/M ) of polymer as indicated at curves. The dotted line is extrapolated for a monodisperse polymer fraction...

Different types of LCB are distinguished. Star polymers are the simplest branched polymers because they have only one branch point. Regular star polymers have a branch point with a constant number (functionality,/) of arms and every arm has the same molecular weight. They are therefore monodisperse polymers. Star polymers may also have arms with a most probable distribution [5], Star polymers can also be polydisperse due to a variable functionality. Palm tree [6] or umbrella polymers [7] that contain a single arm with different molecular weight (MW) than the other arms are classified under the asymmetric star [8] polymers, see Figure 3.2. 

It has long been reaUsed that the key physics determining the rheology of high molecular weight polymers in the melt state arises from the topological interactions between the molecules [1,2]. This is deduced from observations on many different monodisperse materials that ... 

There are many facets of this study which we feel merit further investigation. In particular it is necessary to consider am extension of the proposed model, which in its present form is confined to the performance of a simple column, to cover the behaviour of any set of columns since it is column sets which are normally used. In addition, it is important to consider the input to the model which should be truly representative of polymers with a molecular weight distribution and not merely a concentration pulse of perfectly monodisperse polymer. In relation to this latter suggestion it would be significant if it were possible to link this model to the very real problem of deconvolution, i.e. the removal of instrumental and column broadening from the observed chromatogram to produce the true molecular weight distri-... 

The reader s attention is especially directed to the new terms uniform polymer and nonuniform polymer which denote polymers composed of molecules that are uniform or nonuniform, respectively, with respect to relative molecular mass and constitution. These terms replaced the widely used, but non-descriptive and self-contradictory terms monodisperse polymer and polydisperse polymer . 

If c and dc/dx are known as a function of x and the measurement is carried out in a theta solvent, the molecular weight M of monodisperse polymers can now be calculated precisely. If the solvent is not a theta solvent, the obtained value of M is an apparent molecular weight from which the true value can be calculated upon plotting 1/M vs. c and extrapolation to c —> 0. For polydisperse samples, one has to insert the average of dc/dx in the above equation, and the thus calculated molecular weight represents a weight-average,... 

The important feature of many polymers is simultaneous presence of distributions in two and several molecular characteristics. Polymers exhibiting multiple distributions are called the complex polymers or complex polymer systems. A detailed discussion of molecular characteristics of polymers and their average values and distributions can be found in numerous monographs and reviews, for example [34,35]. For the present purpose, it is important to repeat that all synthetic polymers and also polysaccharides are polydisperse in their nature. Only mother nature is able to produce macromolecules, for example many proteins, with uniform molar mass. The latter are often improperly called monodisperse(d) polymers. 

If Equation (11) represents the relationship between [77] and M for a monodisperse polymer sample, then the intrinsic viscosity for a polydisperse sample containing weight fraction Wi with a molecular weight Mj will be... 

Figure 4A. Molecular weight distribution of essentially monodisperse polymer.

Steady state compliance for a monodisperse polymer of molecular weight . 

Sakai, M.,Fujimoto,T.,Nagasawa,M. Steady flow properties of monodisperse polymer solutions. Molecular weight and polymer concentration dependences of steady shear compliances at zero and finite shear rates. Macromolecules 5,786-792 (1972). 

The polydispersity factor p is evaluated with the aid of any of the well-known viscosity-molecular weight relationships. From eq. (3.60) a proportionality of the intrinsic viscosity to the half power of the molecular weight is expected, as this theory holds for 0-solvents. However, based on the conclusion of Section 3.5, viz. that the reduced steady-state compliance of a monodisperse polymer is insensitive to the excluded... 

The mechanism of the polymerization of this monomer has been studied in far greater detail than any other. It is clear from the outset that a much more complex mechanism is involved than is the case for olefins. A large proportion of the initiator is used to form polymer whose molecular weight is only a few hundreds and the overall molecular weight distribution is so broad as to be rivalled only by those found in polyethylene produced by the high pressure process (19, 39). The initiator disappears almost instantaneously on mixing the reactants (19, 38). Under these conditions, an almost monodisperse polymer would be expected if chain transfer or termination processes are absent. 

We have already mentioned that a generating function contains the same information as the corresponding probability distribution, and the distribution can be obtained by special expansion techniques from the p.g.f25,27-291 84,132-1345 The same technique of expansion can also be applied to the path-weight generating function and yields the properties of molecularly monodisperse fractions of the branched polymer. We will give an example later in Chap. D.I. (See also Appendix). 

It has been found for some systems, and may be true for all, that there is no transition directly from the isotropic to the nematic phase as the critical condition is attained. Instead, a narrow biphasic region is found in which isotropic and nematic phases co-exist. This behaviour was predicted by Flory 2), even although his initial calculations related to monodisperse polymers. It is accentuated by polydispersity (see Flory s review in Vol. 59 of Advances in Polymer Science), and indeed for a polydisperse polymer the nematic phase is found to contain polymer at a higher concentration and of a higher average molecular weight than the isotropic phase with which it is in equilibrium. 

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